The present invention relates to the reduction of ion migration in ring laser gyroscope blocks. Specifically, it relates to reduction of lithium ion migration from the ring laser gyroscope block to gyroscope components or component seals by adding a dielectric barrier material layer which is resistant to ion migration between the gyroscope block and one or more gyroscope components.
Ring laser gyroscopes are formed of solid blocks of material that, over a wide temperature range, are resistant to expansion. Several proprietary materials, such as the material known by the trademark "Zerodur", exhibit excellent dimensional stability over wide temperature ranges. The stability is found to be related to the presence of a lithium oxide in the material. The mentioned Zerodur, for example, includes approximately 3.7% lithium oxide. Unfortunately, lithium is in the form of ions which can be highly mobile in the presence of an electric field. Prevention of lithium migration caused by this field is critical since such migration will cause formation of undesirable lithium-rich layers which weaken component seals and shorten gyroscope life.
The mechanism which leads to the electric field is as follows. Laser gas within conduits in the gyroscope block is excited by placing voltages between spaced-apart anodes and a cathode which are attached to, or embedded in, the gyroscope block. The anodes are at a different potential from the cathode and serve as the primary means for production of an electric field in conjunction with the cathode during gyroscope operation. Other metallic or conductive components attached to, or part of, the gyroscope block may also be at a different potential from the cathode. These components may become secondary means for producing an electric field in the gyroscope block during operation, thus also causing ion migration.
In the case of Zerodur, the electric field created in the gyroscope block during operation applies a force on the lithium ions, attracting them toward the cathode. Oxide ions, on the other hand, migrate toward the anode or other component having a different charge from the cathode. Thus, over the operating life of the gyroscope, a significant amount of lithium ions migrate through the gyroscope block toward the cathode. The lithium ions arriving at the cathode are neutralized by electrons there; hence lithium accumulates at the cathode seal surface. The integrity of the cathode-gyroscope block seal is endangered by this accumulation because the lithium-rich structure is not a mechanically strong one, and the higher the concentration of this material at the seal, the larger the chance of seal failure. The problem may be complicated by non-ideal lithium concentrations in the seal area which cause coefficient of thermal expansion (CTE) variations which may also stress the seal area.
Lithium migration and/or contamination has been recognized as a possible problem in the ring laser gyroscope field, and several solutions have been proposed. In U.S. Pat. No. 5,098,189 to vonBieren, a gas gap is used to interrupt the flow of current in a direct line between the cathode and the anode or other differently-charged components. This solution will only slightly reduce lithium migration since the electric field through the gyroscope block remains, and the lithium migration path is only lengthened slightly. At best, it results in a space charge at the gas gap surface which reduces the electric field in the block. In U.S. Pat. No. 5,432,604 to Canfield, et al., a dielectric material is used to block lithium migration to the laser passages in the gyroscope block because the laser beams tend to carry the lithium and deposit it on the gyroscope mirror surfaces. Neither patent recognizes that significant reduction in lithium migration can be achieved by substantially reducing the electric field appearing in the block of the gyroscope.